Polyurethane foams and glass wools have been widely used as building construction materials for their good thermal insulation qualities. However, because these materials are pathogenic to human body, alternative foamed materials that are compatible with human health have been the focus of significant industrial interests and extensive research effort.
Foamed aluminum alloys have been developed as a result of these efforts. Foamed aluminum alloys are made by adding a foaming agent into a molten aluminum alloy, so as to form a porous metal body containing numerous air bubbles. The specific density of the foamed aluminum alloys is typically between 0.3.about.0.5, with a thermal conductivity of about 0.44 Kcal/m-hr-.degree.C. The foamed aluminum alloys also have another advantage in that, because it is highly porous, it provides good sound shielding effect. Furthermore, because the surface of the porous structure is covered with a layer of aluminum alloy oxide, the actual melting point of the foamed aluminum alloys can be elevated to as high as 1,200.degree. C. Therefore, the foamed aluminum alloys will not melt even when they are subject to temperatures greater than the normal melting point of the constituent aluminum alloys of about 660.degree. C.
With the above mentioned advantages, such as light weight, good heat insulation, good sound absorbing quality, and high heat resistance, coupled with their relatively low cost, foamed aluminum alloys should be an excellent candidate for use as a building construction material. However, these advantageous properties of foamed aluminum alloy plates have not been realized, mainly because these materials do not provide adequate strength so as to allow them to be used in meaningful applications. At the present time, the foamed aluminum alloys have only a very limited number of sporadic applications.
Japanese Laid Open Patent Application 1-275831 discloses a method to improve the strength of foamed aluminum alloys by applying an adhesive layer to attach one of the foamed aluminum alloy plate with a thin aluminum plate to form a foamed aluminum alloy composite (layered) plate. Such an adhesion is not very reliable, especially in high humidity and/or high wind environment, wherein these composite materials are expected to be subject to.
Japanese Laid Open Patent Application 3-33060 discloses a method, by which molten aluminum is first poured into a mold before the foamed aluminum alloy is placed inside, so as to form a composite (also layered) foamed aluminum alloy plate containing the foam aluminum alloy plate with an aluminum layer attached thereto. This method does not require an adhesive and thus provides a more reliable bonding between the foamed aluminum alloy layer and the aluminum reinforcement layer. However, since this method is conducted at very high temperature, many technical difficulties are expected especially in mass productions.
Japanese Laid Open Patent Application 2-229997 discloses a method, by which a foamed aluminum alloy plate and a thin plate are joined together, and then a polyurethane resin is caused to infiltrate into the cavities in the form of bubbles in the foamed aluminum alloy plate, so as to provide an adhesion therebetween. The adhesion provided by this method also is not very reliable, especially in harsh environment, such as the high humidity and/or high wind environment described above.